Archived - Mac OS X v10.6: DNS search order behavior
In Mac OS X v10.6 and later, the search order of DNS servers specified in Network preferences is dynamic, so that servers that don't respond are moved to the end of the search order. This provides performance and reliability improvements over previous Mac OS X versions, but in some configurations it can lead to unexpected results in Mac OS X v10.6 through 10.6.2.
If you have more than one DNS Server specified in the Network pane of System Preferences, and you encounter intermittent issues resolving domain names, update to Mac OS X v10.6.3 or later.
In Mac OS X v10.6, the default DNS server searching behavior is that when a server does not return a result or when it returns SERV_FAIL for a query, and other servers are available to query, the server is temporarily disabled in the search order.
If there is more than one server for the query and all of them have returned SERV_FAIL, the servers will be queried in the order that they were disabled (that is, the server that has been disabled the longest will be used first).